1. Field of the Invention
The present invention relates to a wireless communication scheme, and more particularly, to a communication system and related method for directly transmitting data of a mobile station to another mobile station through a relay device without going via a base station.
2. Description of the Prior Art
For a communication system complying with the IEEE 802.16× standard, such as the Worldwide Interoperability for Microwave Access (WIMAX) system, the essential framework of the communication system is to utilize a base station to transfer data between mobile stations served by the base station itself. If two mobile stations communicate with each other in this condition, data transmitted by either one of the mobile stations will pass through the base station; the data is transmitted according to a point-to-multipoint mode. In order to achieve throughput enhancement and coverage extension, in the IEEE 802.16j specification, it is mentioned that multiple relay stations can be further used between a base station and mobile stations for conveying data. For the wireless network topology formed by the base station, relay stations, and the mobile stations, control signaling and data transmission is usually achieved according to a tree model. When the relay stations are used for achieving throughput enhancement and coverage extension, both control signaling and data actually transmitted in this transmission scheme is controlled via the base station, and a situation where the data is transmitted between the mobile stations in private can be avoided. An advantage is that the base station can directly control the transmission scheme once service provided by the transmission scheme is charged or the transmission scheme needs to execute service interruption. In this situation, only the base station has the capability of service destruction.
If all the transmitted data passes through the base station, the bandwidth resource of the communication system, however, will be greatly wasted. For example, if a mobile station MS1 transmits an amount of data to a mobile station MS2, then this amount of data may be transmitted to a base station through a certain relay station on the uplink associated with the data. It is possible that the data is transmitted to the same relay station from the base station on the downlink and then the data is transmitted to the mobile station MS2 from this relay station. As mentioned above, it is evident that the bandwidth and computation resource between the relay station and base station are occupied twice when the data is transmitted via the uplink and downlink. To solve this problem, if the above-described relay station directly transfers the data to the mobile station MS2 after receiving the data from the mobile station MS1 for preventing system resources from being consumed by the base station, this, however, does not comply with the originally developed standard: data communication between mobile stations are centrally controlled by the base station. Therefore, an acceptable method is that signaling data for control signaling is still controlled by the base station, but bearer data for data transmission can be directly transmitted from one mobile station to another mobile station through relay station(s) without going via the base station. The reason is that the signaling data in general does not occupy excessive bandwidth resources while the bearer data may be multimedia data and therefore occupies more bandwidth resources.
For a WIMAX communication system, another serious problem, however, will result from using relay station(s) to transmit data without going via a base station. The problem is that a mobile station receiving the data cannot decrypt this data. This is because the WIMAX system supports end-to-end security, i.e., each mobile station and one base station has a shared secret key but a plurality of intermediate relay stations do not have the shared secret key. Originally, if all data (i.e. signaling data and bearer data) passes through the base station, the mobile station MS1 encrypts the data with key KEY1 and the base station can decrypt the data with key KEY1. The base station then encrypts the data with another key KEY2, and the mobile station MS2 can decrypt the data with the key KEY2. If, however, the data is directly transmitted through the relay station(s) without going via the base station, the mobile station MS1 encrypts the data with the key KEY1, but the mobile station MS2 cannot decrypt the data because the mobile station MS2 does not obtain the key KEY1 (the mobile station MS2 only has the key KEY2). That is, the data cannot be decrypted although it can be correctly received. The base station may share the key KEY1 with the relay station(s) so that the relay station(s) can use the key KEY1 to decrypt the data transmitted by the mobile station MS1 and then use the key KEY2 to encrypt the received data so that the mobile station MS2 can decrypt the data transmitted from the relay station(s) with key KEY2. However, a more complicated scheme will be required to synchronously update the keys shared between the base station and relay station(s). Of course, this breaks end-to-end security provided by the WIMAX communication system.